Process for corrosion control in boilers

ABSTRACT

A corrosion control process is described. The process is especially useful in the control of chloride corrosion in waste to energy boilers. Corrosion of high temperature surfaces can be assessed by the monitor and controlled introduction of treatment chemicals by targeted in furnace injection reduces corrosion while maximizing combustion efficiency. A corrosion monitor is also described. Before and following selection of corrosion control chemicals and the locations for targeted in furnace injection, injection regimen and chemical selection and introduction parameters are monitored with the aid of the method and apparatus of the invention to adjust one or more control parameters to reduce corrosion. A preferred method will employ a treatment chemical that comprises an SO 2  or SO 3  reagent, e.g., sulfuric acid, sulfur, a sulfate salt or a bisulfite salt.

RELATED APPLICATION AND PRIORITY CLAIM

This application is related to and claims priority to prior U.S. Provisional Patent Application No. 60/681,786 filed May 17, 2005, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a corrosion control process, which is especially useful in the control of chloride corrosion in boilers, particularly waste to energy boilers.

Over several recent years the literature has extensively reported that chloride induced corrosion of high temperature surfaces in waste to energy (WTE) boilers is one of the most costly problems in the industry. This problem can result in replacement of superheater pendants as often as annually in some units or the costly use of higher alloyed materials to either shield the metal surfaces or serve as replacement tube material.

The cost-effectiveness of the replacement alloys has not been proven in many cases, and the industry has been looking for alternative solutions. There is a need for chemical solutions to the problem of corrosion in boilers of all types and especially in the high temperature flue gas near WTE superheater pendants.

The problem is not limited to WTE boilers. In U.S. Pat. No. 6,478,948, Breen, et al., indicate that until recently, furnace boiler tubes corroded slowly and had a service life of 20 to 30 years, but the introduction of low NO_(x) burners has increased the rate of boiler tube corrosion and can reduce their life expectancy to only 1 to 2 years. Breen, et al. point out that the corrosion of furnace wall tubes involves several mechanisms. First, the say that the removal of the protective oxide film allows further oxidation. Second, they say that if the oxide film is not present, the iron surface is attacked and pitted by condensed phase chlorides which may be present. They also point to a third mechanism which occurs when wet slag runs across the surface of the film. As that happens, iron from the tube goes into the slag solution which contains low fusion calcium-iron-silicate eutectics that are formed in the liquid slag under reducing conditions in the furnace. They state that reduced sulfur in the form of S, H₂S, FeS or FeS₂ can react with the oxygen of the tube scale depriving the tube metal of its protective layer.

While the problems of boiler corrosion are well documented and there is a growing understanding of the causes, the available solutions to these problems are not as easily facilitated or economical as would be desired. In a 2004 paper delivered at NAWTEC, Ken Robbins of Maine Recovery Company detailed attempts to use shielding, alternate metallurgies, and various soot blowing strategies to mitigate corrosion found in a WTE unit. The paper also discussed a proprietary chemical slag control program, which was found helpful in controlling slag and minimizing cleaning outages, but had no discernable effect on specific localized corrosion problems. In the case of isolated corrosion, especially on superheater pendant surfaces, which can experience corrosion rates ranging from 0.020 to 0.050 inches per month, tube failures can occur in as little as seven months and create a need for replacement of the entire pendant annually. A photo of a tube removed due to a failure is shown in FIG. 1. An observation that can be made from FIG. 2 is that corrosion is strong on the sides tangential to flue gas flow and occurs on opposing sides of the tube.

A TNO (Nederlandse Organisatie Toegepast—Voor Naturwetenschappelijk) report entitled “Review on Corrosion in Waste Incinerators and Possible Effect of Bromine” provides a mechanistic explanation for the severe corrosion suffered by WTE units. See Ir. P. Rademakers (TNO IND), Ing. W. Hesseling (TNO-MEP), Ir. J. van de Wetering (Akzo Nobel AMC) (July 2002). In addition to the overall analysis of the primary chemical components involved in this corrosion mechanism, it provides a series of equations that may explain why chloride corrosion occurs at the temperature and metallurgical conditions of a waste incinerator.

It is well known that corrosion by high CO levels and reducing atmospheres occurs in the first pass above the grate in-furnace. A refractory lining is often employed on the water walls in the first pass. A strong temperature gradient and condensing substances can also contribute to reducing conditions in these areas. Alkali metal chlorides have been found in deposits near the metal surface, and the high level of chlorides in the waste are strongly implicated with the problem.

Rademakers, et al., explain that high temperature corrosion in waste incinerators is caused by chlorine either in the form of HCl, Cl₂, or combined with Na, K, Zn, Pb, Sn and other elements. Both gaseous HCl with and without a reducing atmosphere and molten chlorides within the deposit, are considered major factors. As with Breen, et al, they point out that sulfur compounds can be corrosive compounds under some circumstances and can influence the corrosion by chlorine.

Rademakers, et al., identify several factors as the most important in high temperature corrosion: the metal temperature and the temperature difference between gas and metal, the flue gas composition, deposits formation and reducing conditions, and the ratio of SO₂/HCl. They indicate that following mechanisms can be distinguished:

-   -   Corrosion by HCl/Cl₂ or SO₂/SO₃ containing gas under oxidizing         or oxidizing/reducing conditions, and     -   Corrosion by solid or molten deposits of metal chlorides and         sulfates.         Rademakers, et al., describe these mechanisms and refer to a         schematic, in FIG. 3, as drawn from Krause, 1986, 1993, and as         set out below in various steps.

Corrosion caused by chlorine-containing gas at metal temperatures above about 450° C. is referred to as ‘active oxidation’. Alkali chlorides, such as NaCl, CaCl₂ and KCl, can be present already or can be formed by the combustion and subsequent reaction of alkali oxides: Na₂O+2HCl=2NaCl+H₂O   [1] Under ideal conditions (good mixing, sufficient residence time) alkali chlorides can be sulfated according to the following reaction, provided there is enough SO₂ and O₂: 2NaCl+SO₂+½ O₂+H₂O═Na₂SO₄+2HCl   [2] This would result in formation of sulfates and volatile HCl. At the relatively low tube wall temperatures of most waste incinerators, the sulfates are not very harmful and the HCl formed will be transported to the flue gas clean up system. However, if the gas reaches the cooler tube walls before the reaction is completed, the alkali metals will tend to condense on the cooler metal. In this case, further sulfate formation can occur on the metal under the release of HCl, and that causes high chlorine partial pressures and enhanced corrosion.

Without SO₂ at 500° C., NaCl and iron oxides can form Cl₂: 2NaCl+Fe₂O₃½ O₂═Na₂Fe₂O₄+Cl₂   [3] 6NaCl+2Fe₃O₄+2 O₂=3Na₂Fe₂O₄+3Cl₂   [4] Calculations of the dissociation constant of HCl as a function of temperature indicate that chlorine is present as Cl₂ under oxidizing conditions up to gas temperatures of 600° C., whereas above 600° C. formation of HCl is enhanced in the presence of water vapor according to the reaction: H₂O+Cl₂=2HCl+½ O₂   [5]

Rademakers, et al., state that at about 500° C., Cl₂ can penetrate pores or cracks in an oxide layer. At the low oxygen partial pressures that exist near the metal-oxide scale boundary, the metal chlorides are the more stable phase. Reactions 3 and 4 can result in a Cl₂ partial pressure sufficiently high that it reacts directly with the steel to form FeCl₂: Fe+Cl₂═FeCl₂ (solid)   [6] The vapor pressures of metal chlorides will depend primarily on the temperature and the HCl content of the gas. In addition, the type of oxide (and alloy) can considerably influence the vapor pressure. The vapor pressure of FeCl₂ is already relatively high at low temperatures. As a result, formation of FeCl₂ can decrease the adherence of the oxide scale or can cause spallation of the oxide layer.

Rademakers, et al., explain that iron chlorides form and migrate out from the corrosion product due to their volatility. At higher oxygen partial pressures near the oxide-gas interface, these chlorides are then converted to oxides and liberate chlorine. These new oxides are not formed as a perfect layer and do not offer protection. Part of the liberated chlorine migrates back through the oxide/deposit to react with the metal at the oxide-metal interface, and form metal chlorides again: FeCl₂ (solid)=FeCl₂ (gas) [7] 4FeCl₂+3O₂═Fe₂O₃+2Cl₂   [8] 3FeCl₂+2O₂═Fe₃O₄+3Cl₂   [9] In this process, the chlorine has a catalytic effect on the oxidation of the metal resulting in enhanced corrosion.

The kinetics of active oxidation is mainly determined by the evaporation and outward diffusion of FeCl₂. Similar chlorine corrosion and regeneration cycles may proceed via FeCl₃ and it is possible for the ferrous iron to be oxidized to the ferric state, which liberates chlorine when oxidized. 4FeCl₂+4HCl+O₂=4FeCl₃+2H₂O   [10] 4FcCl₃+3O₂=2Fc₂O₃+4Cl₂   [11]

The volatility of different compounds can be compared based on the temperature T4 (temperature at which the vapor pressure reaches 10⁻⁴ bar), and vapor pressure values for some compounds are given in Table 1. TABLE 1 T4 Temperatures of Metal Chlorides of Main Alloying Elements Metal chloride T4 (° C.) FeCl₂ 536 FeCl₃ 167 CrCl₂ 741 CrCl₃ 611 NiCl₂ 607

From the above, Rademakers, et al., conclude that low alloy steels and iron-base alloys have limited resistance against active oxidation. High alloyed materials, nickel base alloys in particular, have a much better resistance, which may be because chlorides are more difficult to form and, once formed, have a relatively low volatility. Except for the FeCl₃, most T4 temperatures are well above 500° C. indicating that this mechanism is most relevant to superheaters and less to evaporators.

Corrosion of heat transfer surfaces in boilers has been a major problem, particularly WTE units which generate highly corrosive flue gases, and continues to trouble the industry.

There remains a present challenge to provide a process for taking necessary corrective action to address the corrosion boilers, particularly in WTE units, before damage becomes excessive and requires expensive shut down and repair.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for controlling corrosion of the high temperature surfaces of a boiler, particularly a waste to energy boiler under operating load.

It is another object of the invention to provide a method for reducing corrosion of the high temperature surfaces of a boiler, particularly a waste to energy boiler by the introduction of an inexpensive chemical treatment agent that can modify the corrosion process itself.

These and other objects of the invention are achieved by the invention, which provides a method for corrosion control in a boiler, particularly a waste to energy boiler which involves the introduction of treatment chemicals for the purpose of modulating or preventing the problems of high temperature corrosion of metal surfaces. The invention also provides a new constant temperature probe useful in such a process.

In one aspect the invention will comprise: monitoring the degree of corrosion in a boiler with a corrosion monitor and utilizing the information on corrosion to control introduction of corrosion control chemicals into the boiler. The effect of the chemical on corrosion is monitored and adjusted to provide effective corrosion control. The original placement of the monitor and/or its probes or electrodes and the original introduction parameters for the introduction of corrosion control chemicals are preferably evaluated through the use of computational fluid dynamics.

A preferred corrosion control process of the invention will comprise: disposing an apparatus comprising a constant temperature probe having a corrodible surface in a known position in a boiler; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to control introduction of corrosion control chemicals into the boiler.

In the case where chloride has been identified as a cause of the problem, the preferred chemical treatment provided by the invention is to introduce SO₃ or a precursor of it into the corrosive atmosphere in a manner as to most directly attack the problem, preferably in a targeted fashion. This process applies a source of sulfur directly to the flue gas stream in the location most ideal to drive the reaction toward the sulfate salts. This aspect of the invention provides for the addition of a sulfur compound capable of releasing SO₂ or SO₃, preferably in the form of a sulfate salt, bisulfite salt, sulfur or sulfuric acid, e.g., H₂SO₄, in amounts sufficient to interfere with the chloride chemistry as outlined above and help maintain the chloride in gaseous form.

In one preferred aspect, the method involves subjecting the corrodible surface of said probe to UT measurements, e.g., such as weekly; and then after a predetermined period of time, e.g., 25-30 days, obtaining a metallographic analysis and physical measurement of metal thickness remaining. A preferred apparatus for use in this process will comprise: a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; and temperature sensing means associated with the probe and control means for controlling supply of the fluid to the probe. The probe is adapted to permit insertion and withdrawal from the boiler for visual and physical observation.

One type of apparatus useful in the process of the invention comprises: a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; temperature sensing means associated with the probe; and control means for controlling supply of the fluid to the probe.

A preferred corrosion monitoring process of the invention will comprise: disposing an apparatus as just described in a predetermined operable position in a boiler; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to determine the degree of corrosion of metal surfaces near the predetermined location of the probe to determine when boiler shutdown should be effected for repair and/or cleaning.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its advantages will become more apparent from the following detailed description, especially when taken with the accompanying drawings, wherein:

FIG. 1 is a photograph showing a close-up of rough, wasted surface of a superheater tube covered with friable corrosion product layers.

FIG. 2 is a photograph showing a deep, general metal loss extending to opposite sides of a super heater tube.

FIG. 3 is a schematic outlining the various steps of chemical reactions to help explain corrosion of incinerator boiler tubes.

FIG. 4 is a schematic representation of one embodiment of an apparatus effective in the process of the invention.

FIG. 5 is a schematic representation of a section of the probe described in connection with FIG. 4.

FIG. 6 is a graphical presentation of UT measurements at different times during exposure period.

FIG. 7 is a graphical presentation of remaining wall thickness at different positions on the probe.

FIG. 8 is a graphical presentation of a comparison of treated and untreated UT measurement results.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides processes for monitoring corrosion and for controlling corrosion, which are described below with reference to exemplary embodiments. Because the monitor and the processes are especially useful in the control of chloride corrosion in waste to energy boilers, they will be described in this context while those skilled in the art will see the application of the invention to other environments.

As discussed above, the problem of high temperature corrosion within waste to energy boilers by chlorides is one of the most costly in the industry. The invention enables assessment and control of corrosion of high temperature surfaces and preferably involves the controlled introduction of treatment chemicals by targeted in furnace injection to reduce corrosion while maximizing combustion efficiency. To illustrate the problem of chloride caused corrosion, applicants provide FIG. 1, which is a photograph showing a close-up of rough, wasted surface of a superheater tube covered with friable corrosion product layers, and FIG. 2, which is a photograph showing a deep, general metal loss extending to opposite sides of a super heater tube. The invention provides a method for monitoring this type of corrosion and enables its control to the point that frequent replacement of superheater tubes or constructing of high temperature steel can be avoided.

The chemistry of the corrosion is explained above with reference to FIG. 3, which is a schematic that outlines a probable sequence of chemical reactions to help explain the occurrence and progression of corrosion of incinerator boiler tubes. Where chloride has been identified as a cause of the problem, the corrosion can be controlled by introducing SO₃ or a precursor of it into the corrosive atmosphere in a manner as to most directly attack the problem. In this regard, the teachings of Smyrniotis, et al., U.S. Pat. Nos. 5,740,745 and 5,894,806 and U.S. patent application Ser. No. 10/754072, are instructive of the processing arrangements and control that may be utilized and enhanced with the invention. The monitoring of corrosion and the correction of it by the introduction of SO₃ or precursor chemicals in a targeted fashion can be effective in reducing corrosion and its adverse consequences. The invention provides for the addition of a sulfur compound capable of releasing SO₂ or SO₃, preferably in the form of a sulfate salt, bisulfite salt, sulfur or sulfuric acid, e.g., H₂SO₄, in concentrations and at locations which will interfere with the chloride chemistry as outlined above and help maintain the chloride in gaseous form. An alternative source of SO₂ or SO₃ is Sulfur burner technology. The invention thus can enhance the use of the process and chemicals by carefully applying limited amounts of chemical in a control regimen that will save much greater amounts than would otherwise be spent in superheater tube replacements, materials upgrades and system down time.

Reference is now made to FIG. 4, which is a schematic representation of one embodiment of an apparatus of the invention employing an in-furnace probe 10, and FIG. 5, which is a schematic representation of a section of the probe 10 described in connection with FIG. 4. A preferred apparatus will comprise: a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing it into the probe for cooling the probe; control means for controlling supply of the fluid to the probe; and temperature sensing means associated with the probe and control means for controlling supply of the fluid to the probe. The probe is adapted to permit insertion and withdrawal from the boiler for visual and physical observation.

FIG. 4 shows a preferred form of corrosion monitoring probe 10, which is shown electrically connected to a data logger controller 12 and operationally to a source of cooling air 14. The probe 10 is comprised of an outer tube 16 of a length suitable for inserting it into a boiler from a mount positioned on the wall of a boiler, not shown. The outer tube 16 of probe 10 is preferably made of the same material as the tubes, e.g., superheater boiler tubes presenting the corrosion problem, but can be of an alloy selected based on engineering design to correlate with the properties of the tube metal under the conditions of operation. The outer tube 16 of test probe can be subjected to periodic Ultrasonic Testing (UT) measurements or highly sophisticated (expensive) instruments to measure the corrosivity of the flue gas, instead of waiting for planned outages which may be as much as a year apart. The preferred method of on-line measurement includes exposure of this probe at constant temperature to conditions inside of a boiler at a predetermined position and then removing it periodically, while the boiler remains on-line, for UT measurements. The UT measurements will be at intervals determined by engineering design and can be as frequently as weekly. Then, after a predetermined corrosion target is reached based on UT measurements, or after a predetermined period of time, (e.g., after 25-30 days of exposure) the outer tube 16 of the probe 10 can be withdrawn from the boiler and sent to a laboratory for detailed metallographic analysis and physical measurement of metal thickness remaining.

Another type of corrosion monitor measures electrochemical noise occurring at the surface of the tubes while that surface is exposed to combustion products. U.S. Pat. No. 6,478,948 to Breen, et al., describes such a probe and discusses how it is employed for measuring electrochemical noise and then limiting corrosion by adjusting the ratio of fuel to oxygen—because their premise is that low NO_(x) conditions can result in corrosion. We have determined that such a probe can be useful in corrosion control without altering NO_(x) control measures. The details of the corrosion monitor of Breen, et al., are incorporated herein by reference. The probe is connected to a corrosion monitor having a computer and software which determines a corrosion rate from the measured electrochemical noise. That rate is compared to a standard to determine if the rate is within acceptable limits. If not, the rate and/or location of chemical addition can be changed. As with the probe 10 of the invention, the probe of Breen, et al., can be jacket to control its temperature, preferably to be of constant temperature.

In one preferred mode of operation, the probe 10 shown in FIG. 4 is tested for metal loss and then compared to historical rates of the pendant tubes for purposes of calibration. This data can be used alone or along with visual observations of the surface and impact on the metal surface for purposes of comparison in a control scheme where probe 10 loss is noted, compared to a standard and the result of the comparison used for adjusting chemical treatment to control corrosion. In some cases, the process of the invention will employ a plurality of probes 10. The placement of the probes can be accomplished by computational fluid dynamics, observation and/or trial and error.

The probe 10 is shown to include three thermocouples. Thermocouple 18 is for the purpose of sensing the temperature of the outer tube 16, and is preferably welded to it. A second thermocouple 20 for the purpose of monitoring the temperature of the tip 21 of the outer tube 16 of the probe 10, e.g., for assuring that the tip 21 does not overheat, is positioned a suitable distance, e.g., 1-3 feet from the end of the probe 10. Also important and provided by the invention is a third thermocouple 22, which extends through the wall of the outer tube 16 of the probe 10 at a predetermined location to sense the temperature of the combustion gases in the boiler. The exact number, type and location of the thermocouples will be a matter of design for each individual unit, and the thermocouples can, where feasible, be replaced with other effective temperature sensing means and techniques.

As noted, the probe 10 is maintained at constant temperature. This can be achieved by providing cooling air from controllable source 14, and passing it to the interior of the probe 10 by means of a suitable conduit, e.g., flexible braded hose 24 and suitable couplings, and permitting it to exit the open end of the tube 16. Control valve 26 takes control signals from a controller 12 and supplies air as necessary to maintain the temperature of the probe 10 within a predetermined temperature range. In operation, thermocouples 18 and 20 sense the temperatures at their respective locations and send sensed signals representative of temperature to the controller 12 via suitable lines 28 (or by wireless means not shown). The controller 12 compares the sensed signals to reference values and sends a control signal to the control valve 26 in response to the comparison. Responsive to the control signals, the control valve 26 can adjust the feed of cooling air to provide the amount as needed to the probe 10 to maintain its desired temperature.

The valve 26 can be of any type suitable to provide the control desired, and is preferably an air flow control valve with a digital positioner. The outer tube 16 of probe 10 is attached to probe housing 30, preferably by means, e.g., threaded engagement, which permits easy removal for testing and replacement. The housing 30 also has a fitting 32 to permit ease of connection to air hose 24, preferably for quick coupling and removal. The housing 30 also includes a threaded fitting or other connection means to permit secure attachment to a support mechanism located on the boiler wall (not shown).

The apparatus of the invention as shown in FIG. 4 is useful in gathering data that can be used to select and change operating conditions for a corrosion control process. It is preferred to operate a probe 10 at a given temperature over a period of time and to measure the corrosion at different longitudinal segments of it at timed intervals over a test period. A series of measurements at different temperatures can be helpful in determining baseline conditions and in setting and/or adjusting control parameters. This can be achieved by using one probe in a series of sequential tests or a plurality of probes in a series of overlapping tests. FIG. 6 is a graphical presentation of UT measurements at different times during exposure period and FIG. 7 is a graphical presentation of remaining wall thickness at different positions on the probe. FIG. 8 is a graphical presentation of a comparison of treated and untreated UT measurement results. This type of data is very helpful in both setting initial operation conditions and adjusting control parameters.

According to the invention, sulfur bearing materials in a water based mixture are targeted for injection to the trouble spots in the boiler and as close to the flame as practical in a form designed to maximize the conversion of the chloride salts to their sulfate forms. The primary chemical reaction is believed to be: 2XCl+SO₃+H₂O+→X2SO4+2HCl

-   -   Where X can be a suitable metal anion, e.g., an alkali metal         such as sodium, potassium or the like.         It is believed that without SO₃ at 500° C., NaCl and iron oxides         present in deposits can form Cl₂ as discussed above. It is         believed important to reduce the presence of Cl₂ near the metal         tubes at temperatures above about 500° C., and the introduction         of an SO₃ reagent is a preferred manner of corrosion control         according to the invention. The preferred SO₃ reagent for         application to deposits as they form is a sulfate salt (e.g.,         ZnSO₄) a bisulfite salt, e.g., sodium bisulfite, sulfur and/or         sulfuric acid, or in-situ or on-line generated SO3 from a         combustion of sulfur, at any concentration suitable for use. It         is possible to use concentrated solutions of sodium bisulfite,         ZnSO₄ and/or sulfuric acid, but the reagent is generally diluted         with sufficient water to permit application to the desired area         in the boiler. Typically, concentrations of from about 10 to 90         weight percent will be employed at a molar ratio sulfur to         chlorine in the combustion gases of from 1:3 to 3:1. Ratios near         stoichiometric based on chlorine are preferred.

Before and following selection of corrosion control chemicals and the locations for targeted in furnace injection, injection regimen and chemical selection and introduction parameters are monitored with the aid of the method and apparatus of the invention to adjust one or more control parameters to reduce corrosion. The processing and chemicals can be of the type described in U.S. Pat. Nos. 5,740,745 and 5,894,806 and U.S. patent application Ser. No. 10/754072, all to Smyrniotis, et al., which are incorporated herein by reference, to reduce the problems with chloride corrosion in waste to energy boilers.

The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence which is effective to meet the objectives there intended, unless the context specifically indicates the contrary. 

1. A process for corrosion control in a boiler which comprises: monitoring corrosion within a boiler having a corrosive atmosphere and introducing treatment chemicals containing SO₂ or SO₃ or precursors therefor at positions selected in response to monitored corrosion, to thereby modulate or prevent the problems of high temperature corrosion of metal surfaces.
 2. A process according to claim 1, wherein: a constant temperature probe having a corrodible surface is positioned in a known position in a boiler; the probe is periodically removing for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, the degree of corrosion on the probe is calibrated with what could be expected of the boiler components; and then the observations of the probe as calibrated are utilized to control introduction of corrosion control chemicals into the boiler.
 3. A process according to claim 1, wherein: the degree of corrosion in a boiler is monitored utilizing a probe and signal processor for observing electrochemical noise in a probe within the furnace and comparing that information to known values to provide a signal that corresponds to corrosion rate.
 4. A process according to claim 3, wherein: the probe is a constant temperature probe.
 5. A process according to claim 1 wherein the boiler is a waste to energy boiler.
 6. A process according to claim 1, wherein: probes for a corrosion monitor are placed in original positions and the original introduction parameters for the introduction of corrosion control chemicals are based on computational fluid dynamic determinations.
 7. A process according to claim 1, wherein: a chemical composition comprising an SO₂ or SO₃ or precursor of either is introduced into the corrosive atmosphere in a manner effective to maintain chlorine gas in gaseous form and the sulfate salts of the metal being corroded in solid form.
 8. A process according to claim 7, wherein: the chemical composition comprising SO₂ or SO₃ or precursor of either comprises a sulfate salt, bisulfite salt, sulfuric acid, or sulfur in amounts sufficient to reduce corrosion.
 9. A process according to claim 8, wherein: the corrodible surface of said probe is subjected to periodic UT measurements and to metallographic analysis and physical measurement of metal thickness remaining.
 10. A process according to claim 1, wherein: corrosion is monitored by use of a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; and temperature sensing means associated with the probe and control means for controlling supply of the fluid to the probe; wherein the probe is adapted to permit insertion and withdrawal from the boiler for visual and physical observation.
 11. A method for monitoring corrosion on a real time basis and under operational conditions for boilers, where high temperature corrosion is a problem, comprising: disposing an apparatus comprising a cooled, probe having a corrodible portion with means to sense the temperature of the probe and the surroundings in a predetermined operable position in a boiler; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to determine the degree of corrosion of metal surfaces near the predetermined location of the probe.
 12. A method according to claim 11 wherein the boiler is a waste to energy boiler.
 13. A method according to claim 11 wherein corrosion observed by use of the probe are used to set or adjust introduction of treatment chemicals for the purpose of modulating or preventing the problems of high temperature corrosion of metal surfaces.
 14. A method according to claim 11 wherein the treatment chemical comprises SO₂ or SO₃ or precursor of either.
 15. A process according to claim 14, wherein: the chemical composition comprising SO₂ or SO₃ or precursor of either comprises a sulfate salt, bisulfite salt, sulfur or sulfuric acid, in amounts sufficient to reduce corrosion.
 16. An apparatus of the invention provides a constant temperature metal sample that can be exposed to operating conditions in a WTE boiler and removed on-line and be subjected to UT measurements frequently, e.g., such as weekly and then after a longer period of exposure, e.g., 25-30 days, be sent to a laboratory for detailed metallographic analysis and physical measurement of metal thickness remaining.
 17. A corrosion control process for monitoring corrosion on a real time basis and under operational conditions for boilers, where high temperature corrosion is a problem, comprising: determining initial placement of one or more probes as described in claim 5 in a operable position in a boiler and initial introduction parameters for the introduction of corrosion control chemicals through the use of computational fluid dynamics; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to control introduction of corrosion control chemicals into the boiler.
 18. A method according to claim 17 wherein the treatment chemical comprises SO₂ or SO₃ or precursor of either.
 19. A process according to claim 18, wherein: the chemical composition comprising SO₂ or SO₃ or precursor of either comprises a sulfate salt, bisulfite salt, sulfur, or sulfuric acid, in amounts sufficient to reduce corrosion.
 20. An apparatus for determining corrosion in a boiler, comprising: a probe capable of being fixed to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; temperature sensing means associated with the probe; and control means for controlling supply of the fluid to the probe. 